1. Field of the Invention
This application relates to the field of electronic sensors and more particularly to the field of capacitive sensors for measuring flow and pressure.
2. Description of Related Art
A capacitive sensor for measuring pressure and flow of liquids and gases typically includes a first electrode located remote from a second electrode, with the first and the second electrodes forming a measurement capacitance. At least the first electrode is formed as a membrane which can be deformed by a pressure applied to the membrane.
A sensor of this type is described in DE 33 10 643 and can be employed for measuring both absolute and relative pressures. The sensor, however, cannot provide any additional and/or more detailed information. It is therefore desirable to provide a capacitive sensor which can provide differentiated information about a measurement quantity.
In general, according to one aspect of the invention, at least one electrode of a sensor of this type is formed with a spatial structure to allow measurements providing spatial resolution.
By measuring a spatially differentiated capacitance value or capacitance values of the electrodes, a significantly improved differentiated information about measurement quantity or also about the condition of the sensor itself can be obtained. For example, a flow profile of a fluid or a gas flowing through the measurement capacitor can be determined and also displayed based on the spatial distribution of the capacitance. In addition, such an arrangement also provides improved information about the relative spatial arrangement of the electrodes, since any deviation from a mutual parallel alignment or a relative parallel displacement of the electrodes relative to one another can be readily observed and can therefore be taken into account when evaluating the measurement signals. Furthermore, the spatially resolved measurements and the detected relative position or change in position of the electrodes can also be used to recognize fatigue of the capacitive sensor; to assess the reliability of the measurement itself, and to also estimate the remaining lifetime of the sensor. The sensor of the invention can then be replaced with a functional sensor before a malfunction may indeed occur.
With the spatially structured electrode which includes a plurality of electrode elements that are spaced apart from each other and arranged in a two-dimensional pattern, the measurement results can be spatially resolved. By arranging the electrode elements in the two dimensions, the measurement results can be spatially resolved not only in one direction, but in a plane that is defined by the surface of the substrate. The individual electrode elements may be advantageously arranged in form of a checkerboard pattern and may be identical with respect to their physical properties (size, material, and so on).
The measurement signals derived from the individual elements are essentially identical if the counter electrode has comparable characteristic properties, so that the signals can be easily evaluated to provide the desired spatial resolution. Consequently, the difference in the measurement signals is essentially determined by the spatially differentiated structure of the quantity to be measured and/or by the spatially changing structure of the sensor itself. Although the surface area of the elements is reduced by the two-dimensional arrangement of the electrode elements, causing a proportional reduction in the measurement capacitance, this disadvantage can be overcome by a suitable choice of the signal amplifiers and/or the electronic processing unit. This disadvantage, however, is acceptable, if a two-dimensional spatially differentiated measurement signal is desired.
According to one advantageous embodiment of the invention, the spatially structured electrode is formed of several mutually parallel stripes. By arranging the differentiated spatial structure in the form of parallel stripe-shaped elements, a spatial resolution corresponding to the two-dimensional spacing between adjacent stripe-shaped element can be attained. In addition, the surface area and therefore also the capacitance of the stripe-shaped elements can be made quite large, thereby improving the reliability of the measurement signals. The capacitance is proportional to the area of the stripe-shaped element. Consequently, the capacitive measurement signal may subsequently require only a small amount of signal amplification, which improves the signal-to-noise ratio of the spatially differentiated measurement signal.
According to another advantageous embodiment of the invention, the spaced-apart elements arranged in a two-dimensional pattern may have the form of circles or rectangles. With this arrangement of the electrode elements, the elements can be closely spaced to provide a large surface coverage. This would not be possible if the shape of each element were selected individually. Circularly or rectangularly shaped electrode elements arranged in a two-dimensional arrangement therefore advantageously have large capacitance values which improves the signal quality and the overall reliability of the measurements.
According to yet another embodiment of the invention, an electronic circuit for processing the measurement signals is integrated with one of the substrate members, wherein the integrated circuit is preferably located underneath the electrode of the respective substrate member. With this integration, in particular in the vertical direction, of the capacitive sensor with the electronic circuit for processing the measurement signals, the space taken up by the sensor is used much more efficiently. Consequently, the overall footprint of the system decreases proportional to the increased packing density of the entire system, which includes the capacitive sensor and the electronic processing unit. In addition, the length of the signal path that the measurement signals of the capacitive sensor have to travel, is also reduced. This feature reduces possible interference and improves the accuracy of the quantities to be measured and processed by the capacitive sensor.
According to yet another advantageous embodiment, the integrated electronic circuit includes devices which can separately process the measurement signal having the spatial resolution. The devices are preferably arranged in close proximity to the individual electrode elements of the spatially structured electrode. In this way, the spatially resolved signals can be processed with devices that are identical or at least substantially identical, so that the measurement signals can be processed by a cascaded electronic system. The cascaded arrangement can be expected to reduce not only the costs of developing such a signal processing system, but also the expenses associated with a potential malfunction of the spatially resolving processor units, since this system is more complex and may fail more frequently than conventional capacitive sensors. Moreover, a common central processor that receives the signals processed by the individual devices, can be used for commonly processing the measurement signals, since each of the measurement signals is generated by a processing unit having an identical or essentially identical processor characteristics. The close proximity between the electrode elements and the processing devices also minimizes possible signal losses. Consequently, the decrease in the surface area of the individual electrode elements is not significant in view of the ability to produce a spatially differentiated measurement signal.
According to still another embodiment of the invention, the electronic circuit and/or devices capable of processing signals with a spatial resolution may include amplifier devices that are preferably arranged proximate to the electrode elements of the spatially structured electrode. In this case, the un-amplified measurement signal travels only a very short signal path to the respective signal amplifier or signal processor associated with the element. The amplified measurement signal, on the other hand, which is less susceptive to interference, can be routed to a distal electronic circuit for processing, without suffering a large signal loss. The distal electronic circuit is preferably implemented as a central electronic circuit serving a plurality of amplifier devices.
According to yet another advantageous embodiment, the second substrate member of the sensor in the region of the electrode is formed as a deformable membrane, with the electronic circuit integrated in the first substrate member. In this way, the electronic circuit is not affected when the membrane is deformed during the measurement. Separating the electronics in this way also improves the long-term stability of the sensor, since a deformation of the membrane in the region of an electronic circuit that is integrated in the region proximate to the membrane, can cause microscopically small cracks, which may damage the electronic structures inside the substrate member and cause the sensor to fail.
According to yet another embodiment of the invention, a first portion of the electronic circuit for processing the measurement signals may be located in the first substrate member and a second portion of the electronic circuit is arranged in the second substrate member. The electronic circuit can thus be divided among the two substrate members, so that the electronic circuit integrated with the capacitive sensor has a significantly improved functionality. This is of particular importance when the measurement signals are spatially resolved with a higher resolution. Advantageously, the devices for spatially resolved processing of the spatially resolved measurements may be arranged in the first substrate member which includes the spatially structured electrode, whereas the remaining electronic circuit for commonly processing the measurement signals may be located in the other substrate member. This arrangement tends to further reduce the interference between the different portions of the electronic circuit that are housed in the different substrate members.
According to yet another advantageous embodiment of the invention, at least one of the electrodes or at least a portion of the electrodes may be formed of a conductor path of the electronic circuit that processes the measurement signals. In this way, the electrodes do not require separate conductive surfaces in addition to the existing conductor paths of the electronic circuit, thereby eliminating the need for an additional metalization plane on the substrate member during the manufacture of the sensor. By synergetically using the conductor paths of the electronic circuit also for the electrodes, the internal component structure of the sensor of the invention can be optimized.
According to another advantageous embodiment of the invention, the sensor may include a damping system that is capable of preventing oscillations of the membrane at least to a degree where oscillations of the membrane do not adversely affect the measurement result. An oscillation of the membrane can be damped by filling a region of the moveable membrane with a material which resists a rapid change in position of the membrane that occur during a brief time interval. This can be achieved, for example, by filling the region with a liquid with a predetermined, not too high viscosity. The membrane may also be equipped with reinforcements or ribs, which prevent the membrane to oscillate in a predetermined frequency range by shifting the resonance frequency of the membrane into a frequency range that is substantially different from the frequency range of an expected particular applied force. The membrane can also be provided with a permanent magnet, wherein an external magnetic field acting on the magnet may counteract a deflection of the membrane caused by an oscillation. Such a damping is particularly advantageous when the spatial distribution of the measurement signal is to be analyzed, since elimination of oscillations increases the accuracy and resolution of the. measurement signal. Other characteristic parameters, such as the expected residual lifetime of the sensor and the quality of the spatially differentiated measurements results and the like, can also be more reliably determined.
According to yet another advantageous embodiment of the invention, the sensor may be made of the semiconductor material, in particular, silicon. The capacitive sensor and the integrated electronic processing devices can then be most advantageously implemented. In addition, the advantageous mechanical properties of silicon facilitate formation of the membrane. The sensor may be formed as a pressure, force or acceleration sensor. A pressure, a force or an acceleration deforms the membrane and causes a displacement of the electrode position. The direction and the type of the external force that acts on the membrane, affects the spatial distribution of the measurement signal. For example, a force with a direction normal to the membrane does not cause a lateral displacement of the membrane, whereas a force that attacks at an angle, may cause a lateral displacement of the membrane. The measurement signal is therefore differentiated, since different elements of the spatially structured electrode contribute to the formation of the measurement signal. Unlike conventional pressure sensors based on capacitive sensors, the portion of the pressure that is received by the second substrate member and does not cause the distance between the electrodes to change, can now be evaluated and taken into account during signal processing. Based on the spatially differentiated information, the actual magnitude and direction of the force applied from the outside can be determined. The sensor of the invention therefore provides much more differentiated measurements.
According to another advantageous embodiment of the invention, the capacitive sensor can be used for sensing magnetic fields. Such a magnetic field sensor may include a permanent magnet disposed in the region of the membrane, wherein the position of the magnet changes in response to an external magnetic field, the field strength of which is to be determined. The magnitude and the direction of the magnetic fields can be determined based on the spatially resolved information and can be outputted or displayed as a magnetic field strength.
The spatially resolved measurement is also capable of resolving and displaying spatially differentiated patterns between the electrodes that form the measurement capacitance. In this way, the flow profile of a liquid or gas flowing through the capacitive sensor can be determined. The flow velocity and/or the total flow can also be determined by measuring the pressure changes along the flow path of the material through the device.
Further features and advantages of the present invention will be apparent from the following description of preferred embodiments and from the claims.